1. Technical Field
The present invention relates to oscillators and oscillator arrays.
2. Related Art
When certain molecules adhere to the surface of a crystal oscillator that is in a resonance state, the resonance frequency changes according to the adsorption amount (weight). A technology using such a phenomenon is called QCM (Quarts Crystal Microbalance), and used in sensors for detecting the presence or the amount of molecules.
Generally, AT-cut crystal vibrators are used for QCM. The AT-cut crystal vibrator refers to a cut crystal substrate in a specific crystal orientation with respect to the quartz crystal axis, and is widely used without being limited to QCM because changes in its temperature coefficient adjacent to room temperature are extremely small and it has excellent temperature stability.
An AT-cut crystal vibrator is made of a crystal plate having excitation electrodes formed on front and back surfaces thereof and operates in a so-called thickness shear vibration mode in which the front surface and the back surface move in opposite directions upon application of a voltage between the electrodes. Its resonance frequency f0 is in reverse proportion to the thickness of the crystal plate at a portion between the front and back electrodes, and generally has the following relation.f0(MHz)=1670/the thickness of crystal plate (μm)
It is known that the relation between changes in the oscillation frequency Δf of a crystal vibrator in the QCM using such an AT-cut crystal vibrator and the mass ΔM adsorbed on the crystal vibrator is expressed by the following Sauerbrey equation.
                              Δ          ⁢                                          ⁢          f                =                              -                                          2                ×                                  f                  0                  2                                                                              ρ                  ×                  μ                                                              ×                                    Δ              ⁢                                                          ⁢              M                        A                                              [                  Equation          ⁢                                          ⁢          1                ]            
Where f0 is a resonant frequency of the crystal vibrator, ρ is the density of quartz, μ is the shear modulus of quartz, and A is an effective vibration area (i.e., generally equal to the electrode area).
QCM can be used in a variety of applications. By forming an adsorption film on an oscillator which selectively adsorbs specified molecules such as odor molecules, QCM can be used for odor sensors that detect specified odor.
For example, Japanese Laid-open Patent Application SHO 63-222248 (Patent Document 1) describes a technology that uses an element having a crystal oscillator with electrodes and adsorption films formed thereon (FIG. 6 in Patent Document 1) for detecting the presence of odor substance saturated in the air atmosphere by means of a change in the frequency. Also, Japanese Examined Patent Application Publication HEI 08-012141 (Patent Document 2) and Japanese Laid-open Patent Application HEI 05-187986 (Patent Document 3) describe technologies for specifying the kinds of odors by using a plurality of crystal oscillators.
As the thickness of crystal oscillators is reduced, their mechanical strength becomes weaker, which makes manufacturing and handling thereof more difficult. In this respect, a technology that uses a crystal oscillator in a so-called inverted mesa structure has been developed, in which a recess is formed in one of the surfaces of the crystal oscillator thereby thinning a portion of the crystal oscillator. Such a technology is described in, for example, Japanese Laid-open Patent Application 2001-308666 (Patent Document 4).
There are cases where the mass of adsorbed substance ΔM is extremely small. In such cases, even when adsorbing substance is adsorbed on the adsorption film, the change in the frequency is very small, so that detection of the substance is not readily made.